SEARCH final project
meeting: Palaeoclimate
theme outcomes
Dr Joelle Gergis, Dr Ailie Gallant, Dr Raphael Neukom
and Prof David Karoly
Placing the 13-year ‘Big Dry’ in a long-term
context
?
Recent decadal-scale drying – is it
unusual in a longer-term context?
– Only have 100 years of instrumental data
– Difficult to assess decadal-scale variability using short time series
The role of palaeoclimatology
Tree growth rings
Biological and geological indicators capture natural
climate variability on seasonal–centennial timescales
e.g. tree rings, corals, ice cores, lake sediments, cave
records. Science known as ‘palaeoclimatology’
SEARCH used ‘high resolution’ i.e. monthly–
annually resolved palaeoclimate records to extend the
instrumental climate record centuries into the past
Ice laminations
Provides estimates of pre-industrial
or ‘natural’ climatic variations to
assess recent climate extremes
Palaeoclimate reconstructions are a tool
for comparison with climate models:
Assess the role of ‘natural forcing’ e.g.
solar, volcanic, internal ocean–
atmospheric processes (ENSO, SAM,
IOD) and anthropogenic greenhouse
gas forcing
Coral banding
SEA rainfall reconstruction pilot study (1783–1988)
Corals
Trees
Ice cores
• ‘Proof of concept’ using 12 well-dated, annually resolved records from
Australasia with published climate sensitivity (largely ENSO) to assess the
feasibility of developing a rainfall reconstruction for SE Australia
• Locations sensitive to large-scale climate modes: El Niño–Southern
Oscillation, Indian Ocean Dipole, Southern Annular Mode
Gergis et al. (2012), Climatic Change
Using teleconnections to infer regional
rainfall variations
• Unfortunately there are no records except Tasmanian tree rings are
available from the SE Australian mainland
• Using remote teleconnections to estimate regional rainfall variability
• Can a common climate signal extracted from teleconnected regions
adequately represent variations in southeast Australian rainfall?
Using remote teleconnections to estimate
regional rainfall variability
Can we reconstruct southeast Australian
rainfall using remote observational data?
• Used the Global Historical Climate Network
(GHCN) of observational stations
• Closest to palaeoclimate data locations
• 30 years of data common to all records
• Assessed responses from different variables
– temperature, precipitation, pressure and seasurface temperatures
Reconstructing May–April southeast
Australian rainfall from remote observations
r =0.69
Annual
Variance explained
by the remote
observational
network
47%
r =0.92
Decadal
85%
Gergis et al. (2012), Climatic Change
Capturing large-scale, coherent rainfall variations:
(i) instrumental and (ii) palaeoclimate data
GHCN
observations
Palaeo records
-Grid points where a statistically significant proportion of
the variations in May–April Australian rainfall can be
represented by 12 station GHCN network (left) and first 3
Principal Components of the palaeoclimate network (right)
- Palaeo network is able to capture the common signal
seen in the instrumental GHCN station network
Gergis et al. (2012), Climatic Change
Projecting coherent signals onto SEA rainfall
Multiple linear
regression to
project the three
leading palaeo PCs
onto southeast
Australian rainfall
Independent
‘calibration’ and
‘verification’
periods to
determine
regression
coefficients (αn)
x1
rainfall =
x2
α1x 1 + α 2x 2 + α 3x 3
x3
Gergis et al. (2012), Climatic Change
Sensitivity of ‘calibration’ and ‘verification’
periods
Inter-annual calibration and verification metrics
Model skill sensitive
to choice of
calibration and
verification period
Statistic
Definition
ar2
Variance explained adjusted
for degrees of freedom
r
Correlation
RE
Reduction of Error
CE
Coefficient of Efficiency
ST
Sign Test
RMSE
Root Mean Square Error
Gergis et al. (2012), Climatic Change
Histograms of skill metrics provide better
estimates of skill than traditional single estimates
Monte Carlo resampling to generate a 10 000-member
ensemble to better quantify uncertainty:
– select randomised decades for calibration over 1900–1988
– remaining years used for model verification
Gergis et al. (2012), Climatic Change
Ensemble median to represent our ‘best
estimate’ SEA May–April rainfall reconstruction
Captures 33% of
inter annual
variations in
observed SEA rain
Statistic
Definition
ar2
Correlation
adjusted for
degrees of
freedom
r
Correlation
RE
Reduction of
Error
CE
Coefficient of
Efficiency
ST
Sign Test
RMSE
Root Mean
Square Error
rcalibration
0.85 ±
0.15
Median of 10,000 member ensemble captures
72% of decadal variations in instrumental rainfall
ar2calibration rverification
0.70 ±
0.25
0.85 ±
0.19
RE
CE
0.73 ±
0.32
0.66 ±
0.52
ST
80% ±
20%
RMSE
0.56σ ±
0.19σ
Gergis et al. (2012), Climatic Change
SE Australian rainfall variability: 1783 – 2009
Wettest period in instrumental record
(1950s and 1970s)
Driest period in instrumental ?
Rapid changes from wet torecord
dry (and
to wet) 2000s)
(latedry
1990s/early
evident through record
Median of 10,000 member ensemble of rainfall reconstruction
Observed area-averaged rainfall from AWAP rainfall grid (1900–2009)
Observed area-averaged rainfall from nine high-quality stations (1873–2006)
Gergis et al. (2012), Climatic Change
How does the ‘Big Dry’ compare to rainfall
estimations since 1783?
2.9%
According to our rainfall reconstruction ensemble, there
is a 97% chance that the 1998–2008 decadal rainfall
deficit was the lowest since European settlement
Gergis et al. (2012), Climatic Change
Unusual happenings: 1820s and 1830s
Independent records from
Lake George report
maxima in lake levels
during 1820s
– ‘A magnificent sheet of
water’ (H. C. Russell, 1821)
– ‘20 miles long and 8
miles wide’ (H. C. Russell,
1824)
Gergis et al. (2012), Climatic Change
Low frequency ENSO–SE Australian rainfall
relationship
• Comparing low-frequency variations in
reconstructed ENSO to assess the stability of
southeast Australian rainfall
• McGregor et al. (2010) looked for a common
signal in a number of ENSO reconstructions
back to A.D. 1650 – proxy for decadal-scale
variations in the Pacific (IPO)
Interdecadal Pacific variability and southeast
Australian rainfall: 1793–1970
Gergis et al. (2012), Climatic Change
•
•
Breakdown is associated with pronounced wet period 1818–1833 (peaking at ~100mm above C20th average)
is a real climate signal not a data issue (tested individual proxies, combinations and independent ENSO
reconstructions)..decoupling of SEA rainfall–ENSO relationship (seen in other studies overseas)
Pre 1840: early 1800s cooling (1oC colder than present), and period of high tropical volcanism e.g. 1816
Tambora eruption ‘year without a summer’…opportunity for pre-industrial detection and attribution studies
River Murray streamflow reconstruction, 1783-1988
Gallant and Gergis (2011), Water Resources Research
r=0.49
r=0.72
24% of annual variations captured
52% of decadal variations captured
- Nine proxies used for August–July streamflow reconstruction (none from within the MDB)
- Losing reconstruction skill at the catchment level, but the results are promising
- Reconstruction can only capture rainfall component of streamflow so is limited
River Murray streamflow and the IPO
Gallant and Gergis (2011), Water Resources Research
Recent River Murray streamflow deficits in a longerterm context
• Where does the River Murray streamflow deficit
sit in a longer-term context?
River Murray streamflow
Using RECONSTRUCTED
OBSERVATIONS from the 10,000member reconstruction
ensemble we estimate that there
is only a 2.3% chance that the
1998–2009 streamflow deficit
has been exceeded since 1783
(length of the reconstruction)
Also used SIMULATED statistical modeling of River Murray streamflow (100,000
year synthetic simulations based on parameters derived from our 10,000
palaeostreamflow reconstructions) to estimate that the 1998–2009 streamflow
deficit has an Average Recurrence Interval of 1 in 1500 years
Gallant and Gergis (2011), Water Resources Research
Estimating hydroclimatic
variations in
Melbourne’s water
supply catchments using
palaeoclimate data
Ailie Gallant, Joelle Gergis and David Karoly
With thanks to K.S. Tan, Bruce Rhodes and Ted Chylinski
Instrumental and palaeoclimate data
Melbourne Catchment – Maroondah,
O’Shannassy, Upper Yarra and Thomson
S11 streamflow network
iS7 independent streamflow network
R9 rainfall network
Melbourne Catchment Streamflow
Melbourne Catchment Rainfall
Preliminary results: extremes in a long-term context
Extreme streamflows in reconstruction are consistent with magnitude and frequency of observed
streamflows during very wet and very dry years, and very wet decades.
Observed streamflows during very dry decades (i.e. 1997-2007) unusual in reconstruction – 0.5th
percentile.
Average Return Intervals (ARI) sensitive to input parameters – difference in mean of < 5% leads to order
of magnitude difference in ARI
PAGES regional 2K network – Aus2K
- Global effort to consolidate regional palaeo data of the last 2000 years for IPCC AR5
-To produce high-resolution, proxy-based climate reconstructions for comparison with high
resolution Earth System Models
-‘Aus2K’ Australasian component: important area of southern mid-latitudes: El Nino–
Southern Oscillation (ENSO), Southern Annual Mode (SAM), Indian Ocean Dipole (IOD)
Source: http://www.pages-igbp.org/index.html
Availability of palaeoclimate data in the Australian region
IPCC AR4 (2007)
Only 5 annually or
decadally resolve
records available
for the Australasian
region for the last
IPCC report
Neukom and Gergis recently
compiled and reviewed 174
high-resolution records from
the Southern Hemisphere to
improve data availability for
climate analysis
Now over 50 sites from
Australasia available for
palaeoclimate reconstruction
Neukom and Gergis (2012), The Holocene
Development of the Au2K database
- Developed a database containing all monthly–annually resolved palaeoclimate
records from Australasia covering last 2K (inc associated literature)
- All records quality checked, reassessed (e.g. tree ring standardisation, EPS
assessment etc) and reprocessed into ‘ready to use’ format for climate analysis
- Searchable by domain, archive etc then data exported for analysis – research tool
Reassessing the climate sensitivity of the
Australasian palaeoclimate network
-Reassessing Australasian proxies for potential to develop temperature and rainfall
reconstructions: strong covariations in instrumental observations are found in some regions
- Spatial correlation map of WA Callitris tree rings vs. JJASON (winter) rainfall and temperature
- Published based on rainfall sensitivity but the record is clearly responding to winter temp in South
Western Australia and Northern Territory ‘dry’ season
- Identifying temperature, rainfall and ‘both’ proxy subsets for sub regional climate reconstruction
Australasian temperatures of past 1000 years
Gergis et al. (2011), Journal of Climate, in revision.
-
1000 year Australasian spring/summer (SONDJF) mean temperature anomalies (land and ocean): mean of 3000member ensemble based on varying reconstruction parameters (28 proxy records)
-
96.5% of our reconstruction ensemble members indicate that there are no other warm periods in the past
millennium that match or exceed post-1950 warming observed in Australasia
-
Three warmest decade of the past 1000 years occur consecutively in the 1980s, 1990s and 2000s
-
CSIRO Mk 3L model comparison show that internal atmospheric–ocean circulation and anthropogenic forcing have
more influence than solar and volcanic variations in reconstructed Australasian temperatures: greenhouse gas
forcing from 1950 onward is swamping the influence of ‘natural variability’
Palaeoclimate outcomes
•
Palaeoclimatology is a powerful tool for assessing pre-industrial climate variations to
assess recent anthropogenically forced climate change
•
Progress by a small team includes the development of:
1.
SEA Rainfall reconstruction for SEA, A.D. 1783-2009
2.
River Murray streamflow reconstruction, A.D. 1783-2009
3.
Experimental Melbourne Catchment streamflow, A.D. 1783-2009
4.
Southern Hemisphere palaeoclimate database for large-scale climate circulation
reconstruction e.g. ENSO, SAM, IOD
5.
Australasian temperature reconstruction, A.D. 1000–2000
•
Recent 1998–2009 drought and low streamflow periods are anomalous in the
context of the past two centuries – return period of 1 in 1500 years
•
Post 1950 temperatures in the Australasian region are the warmest of the past 1000
years, anthropogenic forcing is now the dominant cause of temperature variations
Publications
1. Gergis, J., Gallant, J. E., Braganaza, K., Karoly, D. J., Allen, K., Cullen, L., D'Arrigo, R.,
Goodwin, I., Grierson, P. and McGregor, S. (2012). On the long-term context of the 1997–2009
‘Big Dry’ in south-eastern Australia: insights from a 206-year multi-proxy rainfall reconstruction
Climatic Change: 111 (3): 923–944.
1. Gallant, A. J. E. and Gergis, J. (2011). An experimental streamflow reconstruction for the River
Murray, Australia, 1783–1988. Water Resources Research 47 (W00G04):
doi:10.1029/2010WR009832
2. Neukom, R. and Gergis, J. (2012). Southern Hemisphere high resolution palaeoclimate records
of the past 2000 years. The Holocene 5: 501–524.
3. Gergis, J., Neukom, R., Gallant, A., Phipps, S.J., Karoly, D.J. and Aus2K members. Evidence of
rapid late 20th century warming from an Australasian temperature reconstruction spanning the
last millennium. Journal of Climate (in revision).
1. Phipps, S., Gergis, J, McGregor, H., Gallant, A., Neukom, R., Stevenson, S., van Ommen, T.,
Brown, J., Fischer, M., Ackerley, D. Palaeoclimate data–model comparison: concepts and
application to the climate of Australasia over the past 1500 years. Journal of Climate (in
revision).
1. PAGES Regional 2K consortium including Aus2K coauthors Gergis, J., Lorrey A.M and Phipps,
S.J. Continental-scale temperature variability over the Common Era, Science (in review).